Many work machines, particularly earth working machines, use a hydrostatic drive system to drive the traction wheels or tracks of the work machine. The hydrostatic drive system can provide a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed can be continuously varied by controlling the displacements of either a hydraulic pump or a hydraulic motor which comprise the hydrostatic drive system.
One problem with earth moving machines which use hydrostatic transmissions is that the speed output can be varied rapidly thereby producing a rapid response. This rapid response can result in high, undesirable jerk which can cause discomfort to an operator of the work machine. In addition, the high undesirable jerk can cause the operator to lose control of the machine as the rapid movement of the machine can create unwanted pedal modulations. The unwanted speed pedal modulations create control signals which cause the work machine to move in an unintended manner.
One solution to this problem is pass the control signals from the speed pedal through a low pass filter. Each low pass filter has a selected corner frequency. One corner frequency can provide optimal response during the application of oncoming jerk, i.e. jerk applied to first start accelerating the work machine, but has poor response during the application of off going jerk, i.e. the jerk applied to match the actual speed to the desired speed. On the other hand, another corner frequency will have good response during the application of off going jerk, but will have poor response during the application of oncoming jerk. A drawback of using a low pass filter is that a single filter cannot be selected which provides satisfactory response during both the oncoming jerk and the off going jerk.
Another solution to this problem is to control the oncoming jerk to provide a smooth and rapid response as the machine is accelerated. However, this solution does not control the off going jerk as the actual speed approaches the desired speed. Smoothing the off going jerk is critical for operator comfort and controllability of the work machine. A drawback to controlling only the oncoming jerk is that the response during the application of off going jerk is unsatisfactorily uncomfortable to the operator.
A third solution to this problem is to provide both oncoming and off going jerk shaping to the velocity command. This solution has been applied to work machines having a discrete number of velocity profiles, or speed targets. However, the discrete number of velocity profiles cannot provide the desired response for an infinite number of velocity values that can be commanded by the continuously variable transmission. This scheme does not allow a continuously variable adjustment of the velocity profile to match the continuously variable speed output of the transmission.
What is needed therefore is a method and apparatus for generating velocity commands for a continuously variable transmission which overcomes the above-mentioned drawbacks.